As a normal human being, you're by now probably tired of something you've done over and overt again but in actuality don't get an extraordinary result in any way. May it be on love, studies, etc etc. When to know when you've had enough? That is the question.
You've probably experienced a thing that is beyond your ordinary tolerance that's why you're asking yourself this. Well, I can say that you haven't had enough. If it happened twice or trice, you can decide for yourself if you've had enough. ENOUGH, in most sense is subjective. Unless otherwise specified. Like a credit card limit or a school project deadline.
EMOTIONS, for example, are the most subjective kind of thingy that you can relate this to. If you've had enough, you'll probably know. If you're hurting, then that's a good sign that you've had enough. If you want to die, that's a clear sign that you've had enough. If that isn't to you, I don't know what is. BUT DON'T LOSE HOPE MY FRIEND.
Human being is most likely subject for recovery. Chances are you'll try and eventually experience the same shit again so I wish you GOOD LUCK. But really. It gets better. At least you'll know what to do.
I say, be wise and know when to stay or when to move-on. There's nothing wrong in believing that things will work-out as you will, but doing it over and over and over and over again (provided you're doing an extra something each time you repeat) but still things aren't going your way. SERIOUSLY? Think it over and plan accordingly. It's either you're doing something wrong or it's probably just the way it is and you can't do anything to change it.
So whatever you're thinking of as of the moment, ask yourself this, "IS IT WORTH ANOTHER TRY?" AND "WHY?" or "WHY NOT?"